8.3.6 GNU and old GNUtar format

GNUtar was based on an early draft of the
POSIX 1003.1 ustar standard. GNU extensions to
tar, such as the support for file names longer than 100
characters, use portions of the tar header record which were
specified in that POSIX draft as unused. Subsequent changes in
POSIX have allocated the same parts of the header record for
other purposes. As a result, GNUtar format is
incompatible with the current POSIX specification, and with
tar programs that follow it.

In the majority of cases, tar will be configured to create
this format by default. This will change in future releases, since
we plan to make `POSIX' format the default.